1. Field of the Invention
The present invention relates to a sheet conveyance device that is equipped in an image forming apparatus like a printer or an image reading apparatus like a scanner, an image reading apparatus and an image forming apparatus that use the sheet conveyance device.
2. Description of the Related Art
Generally, an image forming apparatus such as a copier or a printer conveys a sheet to an image forming unit, and forms (prints) an image on the sheet. When the sheet is conveyed, the sheet may slant (skew) with respect to a sheet conveyance direction. Then, when the sheet skews, the image forming unit cannot print in high accuracy because a toner image is transferred to a deviated position of the sheet. Therefore, a conventional sheet conveyance device is provided with a skew correction mechanism for correcting the skew of sheet.
For example, Japanese Laid-Open Patent Publication (Kokai) No. H4-277151 (JP H4-277151A) discloses a skew correction mechanism of an active registration system that corrects the skew generated when a sheet is fed while conveying the sheet.
Incidentally, an image forming apparatus is required to form images on various types of sheets. For example, there is a demand for forming (printing) an image on a non-rectangular sheet such as an index sheet (it is also called a tab sheet). As shown in FIG. 15A, an index sheet 81 has an index part 81a in an edge side for filling in a title etc. for the purpose of classification. The technique disclosed in JP H4-277151A does not assume use of such a non-rectangular sheet.
Japanese Laid-Open Patent Publication (Kokai) No. 2003-146485 (JP 2003-146485A) discloses a skew correction mechanism as shown in FIG. 15C as a first embodiment. The disclosed skew correction mechanism is provided with skew detection sensors 82 and 83 arranged in a direction intersecting a conveyance direction of a sheet 81, conveying rollers 84 and 85, and a control unit (not shown) that independently controls conveyance speeds of the conveying rollers 84 and 85.
As shown in FIG. 15B, sheet configuration information that presents a dimension X of the index part 81a of the sheet 81 in the conveyance direction and position information that presents a position of the index part 81a in the index sheet are beforehand registered into a memory of the control unit.
An edge of the index sheet 81 is detected by the skew detection sensors 82 and 83. The control unit obtains the skew amount of the index sheet 81 according to the detection result concerned, the above-mentioned dimension X (the sheet configuration information), and the position information. Then, the skew of the index sheet is corrected by controlling the conveyance speeds of the conveying rollers 84 and 85 according to the skew amount.
However, there are several types of index sheets of which index parts are formed at different positions. That is, the index parts of the index sheets are shifted to one another so that a user can easily check titles etc. filled in the index parts when the index sheets are piled up.
In order to correct the skew of such an index sheet, it is necessary to know beforehand whether an index part will pass through the skew detection sensor. Therefore, since a user is required to set position and dimension of an index part to an image forming apparatus in addition to setting a sheet type (an index sheet or a regular sheet), the operation is complicated.
On the other hand, an apparatus in a second embodiment of Japanese Laid-Open Patent Publication (Kokai) No. 2003-146485 (JP 2003-146485A) is provided with a line sensor that consists of CCD's aligned in a width direction perpendicular to the sheet conveyance direction and detects an end shape and skew of a sheet by the line sensor. Then, the skew amount of the sheet is computed by processing an image of the end shape of the sheet detected by the line sensor, and the skew of the sheet is corrected in this embodiment.
Since the end shape of the sheet can be detected by using the line sensor, the user does not need to set the sheet shape beforehand, which reduces a user's load. However, the image processing by arranging the line sensor complicates the configuration and process of the apparatus and increases a cost significantly.
Such a problem also comes up in an image reading apparatus that corrects skew of an original.